Oxide superconducting wire

ABSTRACT

A method of producing an oxide superconducting wire. A non-oxidizing metal layer is formed between an oxide superconducting material and an oxidizing metal support in order to prevent oxygen from being taken away from the oxide superconducting material by the oxidizing metal support during a subsequent heat treatment for producing an oxide superconductor to thereby obtaining a wire composite. The wire composite is then heated to produce the oxide superconductor.

This application is a continuation of application Ser. No. 07/170,018,filed on Mar. 18, 1988, now abandoned.

The present invention relates, though not exclusively, to a method ofproducing an oxide superconducting wire which may be used in, forexample, a power-transmission cable or a superconducting magnet coil,and the oxide superconducting wire produced by this method. In thisspecification, the term "superconducting wire" means a superconductingwire, superconducting tape or wire having a similar shape.

In recent times, various oxide superconductors have increasingly beendiscovered which show very high values of the critical temperatures (Tc)at which the transition from a normal conductive state to asuperconductive state takes place. Since such an oxide superconductorshows a higher critical temperature than conventional alloy orintermetallic compound superconductors, it is considered that oxidesuperconductors will highly promise for practical superconductingmaterials.

However, this sort of superconductor is made of ceramics and is thusvery brittle, cracks being easily produced. We have attempted theproduction of various superconducting wires each comprising an oxidesuperconductor as the conducting portion, in which a metal sheath wascharged with an oxide superconducting material in a powder form, and theoxide superconductor charged metal sheath was then subjected to diameterreduction and then to heat treatment.

In the above-described method, however, the oxide superconductingmaterial powder is in contact with the metal sheath which contains anoxidizing metal such as copper, a copper alloy, or stainless steel,during the heat treatment, in which this oxidizing metal takes oxygenaway from the superconducting material and is oxidized. As a result, thecontact portions between the superconducting material powder and theoxidizing metal lacks oxygen. Consequently, a problem occurs inreductions both in the critical temperature and in the critical currentdensity of the product superconductor.

Therefore, it is an object of the present invention to provide a methodof producing an oxide superconductor, as well as the oxidesuperconductor produced by this method which is capable of reinforcingand protecting the superconductor portion without producing any loss ofperformance of the superconductor produced, particularly with respect toboth the critical temperature and critical current density.

One aspect of the present invention is directed to a method of producingan oxide superconducting wire in which a non-oxidizing metal layer isformed between an oxide superconducting material and an oxidizing metalsupport in order to prevent oxygen from being taken away from the oxidesuperconducting material by the oxidizing metal for a subsequent heattreatment, thereby producing a superconducting material, which is thenheated in order to form an oxide superconductor.

Another aspect of the present invention is directed to an oxidesuperconductor produced by the above-described method.

Since an oxide superconducting wire in accordance with the presentinvention may have a fairly large length and exhibits excellentsuperconductivity, it may be used in, for example, a power-transmissioncable or a superconductive magnet.

In the present invention, the term "oxide superconducting material"means a material containing elements comprising an oxide superconductor,and it contains, for example, alkali earth metal elements, elements inGroup IIIa of the Periodic Table, and copper oxides. Alkali earth metalelements may include Be, Sr, Mg, Ca, Ba, and Ra. Such alkali earth metalelements may be employed in the form of a powder of a compound such as acarbonate, oxide, chloride, sulfide, or fluoride, or an alloy powder.

The elements in Group IIIa of the Periodic Table may include, accordingto the present invention, Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb and Lu. Such elements in Group IIIa of the Periodic Tablemay be employed in the form of a powder of a compound such as acarbonate, oxide, chloride, sulfide, or fluoride, or an alloy powder.

The copper oxide according to the present invention may include CuO, Cu₂O, Cu₂ O₃ and Cu₄ O which may be used in the form of a powder.

When the oxide superconductor of the present invention is typicallyexpressed by the following formula:

    A.sub.x B.sub.y Cu.sub.z O.sub.9-δ

wherein A denotes an element in Group IIIa of the Periodic Table and Bdenotes an alkali earth metal element, x, y, z and δ are typicallywithin the range of 0.1≦×≦2.0, 1≦y≦3, 1≦z≦3, and 0≦δ≦7, respectively. Ina Y-Ba-Cu-O system, it should be an orthorhombic system and it istypically that x=1, y=2, z=3, and 2<δ<3, preferably δ=about 2. In a(La_(2-x) M_(x))CuO₄ system, typically 0<x<0.3 and preferably x=0.15wherein M may include Be, Sr, Mg, Ca, Ba or Ra.

When a powder of a superconducting material is prepared, a materialcontaining one or more of the above-described alkali earth metalelements and elements in Group IIIa of the Periodic Table may beselected in the present invention. For example, superconductor rawmaterials for a La--Sr--Cu--O, Y--Ba--Cu--O, La--Yb--Ba--Cu--O orLa--Yb--Ba--Sr--Cu--O system may be prepared.

In addition, when a powder of a superconducting material contains acarbonate or carbon content, this material powder may be preliminarilycalcined in order to thermally decompose the carbonate or carbon contentin the powder so as to deposit it as a carbide on the inner surface of asheath material. This preliminary calcination is preferably performedunder such conditions that the temperature is lower than the heatingtemperature at which an oxide superconductor is produced, typicallyabout 500° C. to 900° C., preferably about 650° C. to 750° C., and for atime of about 1 to 10 hours. If an analysis performed after thepreliminary calcination shows that the carbonate or carbon contentremains in the powder, the powder is again subjected to preliminarycalcination. The preliminary calcination prevents deterioration in theprocessing property of a superconducting material due to the presence ofcarbon and reduces such troubles as breaking of a wire during thediameter reduction. Thus, wires or strips having a fairly large lengthcan be easily obtained.

After the carbon content in the superconducting material powder has beensufficiently removed by the above-described method, the superconductingmaterial powder may be preferably subjected to temporary calcination.The temporary calcination is performed under such conditions as atemperature of, typically about 500° C. to about 950° C., preferablyabout 850° C. to about 950° C., and a time of about 1 hour to about 30hours. The temporary calcination partically produces reactions betweenthe oxide of the alkali metal element, the oxide of the element in GroupIIIa of the Periodic Table, and the copper oxide contained in thepowder, so that at least part of the powder is changed into an oxideexhibiting superconductivity.

After the thus-obtained powder material has been ground well so that theparticle sizes are made uniform, it may be used in the present inventionin the processing of an oxide superconducting material.

The elongated support of the oxidizing metal in the present inventionmay be made of Cu, Cu alloys, high-melting point metals such as Ta, Nband Mo and stainless steel. This support may be according to the presentinvention employed for the purpose of facilitating the processing of asuperconducting wire and also for achieving the protection and/orreinforcement of a superconductor.

In the present invention, materials that are used for the non-oxidizingmetal layer, interposed between the oxide superconducting material andthe oxidizing metal support, may include noble metals such as Ag, Au,Pt, Ru, Rh, Pd, Os and Ir and noble metal alloys such as Ag alloys andAu alloys. The non-oxidizing metal layer may be formed on the innersurface and/or the outer surface of the oxidizing metal support, whichsurface is in contact with the superconducting oxide material, by meansof a conventional surface treatment method such as a plating method, afilm-forming technique such as a chemical vapor deposition (CVD), vapordeposition, sputtering or dipping method, or a method of winding a noblemetal tape or cladding a noble metal pipe around it, before thesuperconducting oxide material and the oxidizing metal support are puttogether. The non-oxidizing metal layer may also be formed on the outersurface of a molded product formed of the superconducting oxide materialby any of the above-described various methods, and the oxidizing metalsupport is then placed around the molded product through thenon-oxidizing metal layer. The non-oxidizing metal layer, formed betweenthe superconducting oxide material and the oxidizing metal support bythe above-described method, is then subjected to heat treatment in aflow of oxygen gas at the diffusion temperature, typically about 700° C.to about 1100° C., preferably about 800° C. to about 1100° C., for atime of about 1 to 300 hours, preferably about 1 to 100 hours.

Although the rate of temperature rise from room temperature to theabove-described calcination temperature may be according to the presentinvention larger than about 500° C./hour, it is typically 200° C./houror less, preferably about 50° to 100° C./hour. At a rate of temperaturerise smaller than about 200° C./hour, an excellent effect of preventingcracks due to thermal stress is obtained. On the other hand, the rate oftemperature decrease from the calcination temperature to roomtemperature is usually smaller than about 200° C./hour, typicallysmaller than about 100° C./hour and is preferably about 20° to 50°C./hour. At a rate of temperature decrease over about 200° C./hour,cracks due to thermal stress may not be sufficiently prevented fromoccuring.

When the calcined material is cooled, it may be according to the presentinvention maintained for a given time in the temperature range in whichthe crystal form of the oxide superconductor transforms from a cubicsystem to a rhombic system, and it may be then cooled to roomtemperature so that the crystal structure is transformed to a rhombicsystem having superconductivity, whereby an oxide superconductor havinga high critical temperature and high critical current density isprovided. In the Y--Ba--Cu--O system, the crystal structure istransformed to a rhombic system by maintaining the oxide superconductortypically within the temperature range of about 400° C. to about 500° C.for about 5 to 48 hours.

FIG. 1 is a schematic drawing of an embodiment of the present invention;

FIG. 2 is a schematic sectional view of a superconducting wire elementproduced by the method shown in FIG. 1;

FIG. 3 is a schematic sectional view of a modification of thesuperconducting wire element shown in FIG. 2;

FIG. 4 is a schematic sectional view of a composite element which isused in another embodiment of the present invention and comprises amolded product and a non-oxidizing metal layer provided thereon;

FIG. 5 is a schematic sectional view of a composite element assemblywhich is formed by covering the composite elements shown in FIG. 4 withan oxidizing metal pipe;

FIG. 6 is a schematic sectional view of an assembly with a reduceddiameter which is obtained by subjecting the assembly shown in FIG. 5 toa process for reducing the diameter;

FIG. 7 is a schematic sectional view of an element wire formed bycovering the assembly with a reduced diameter shown in FIG. 6 with atube of stabilizing material; and

FIG. 8 is a schematic sectional view of an element wire having a reduceddiameter which is obtained by subjecting the element wire shown in FIG.7 to a process for reducing the diameter.

The present invention is described below on the basis of certainembodiments with reference to the accompanying drawings, but the presentinvention is not limited to these embodiments.

FIG. 1 is a schematic diagram of an apparatus for performing a method ofthe present invention. In FIG. 1, reference numeral 10 denotes afour-direction roll; reference numeral 11, an induction heater; andreference numeral 12, a winding machine. All these devices areconventional ones. A hopper 13 is provided at one side end of thefour-direction roll 10.

A discharge portion at the lower end of the hopper 13 is placed near theroll portion of the four-direction roll 10 so as to be able to supply apowder 15 of the superconducting oxide material which is received in thehopper 13 onto the periphery of a wire passing through the roll portion.

The powder material 15 is a mixed powder obtained by mixing at a givenratio powders of one or more elements of the above-described elements inGroup IIIa of the Periodic Table or of one or more compounds of theelements in Group IIIa (for example, oxide powders, chloride powders, orcarbonate powders), powders of one or more elements of theabove-described alkali earth metal elements or one or more compounds ofthose alkali metal earth elements, and a powder of a copper oxide suchas CuO in a given ratio, or a calcined powder obtained by heating such amixed powder at about 500° C. to 700° C. The powder material 15 may alsobe a mixture, comprising a previously produced superconducting oxidepowder, mixed with the above-described powder.

When a method of the present invention is performed by using theapparatus shown in FIG. 1 to produce a superconducting wire, areinforcing core material rod 20, which serves as an oxidizing metalsupport and comprises Cu or a Cu alloy, a high-melting point metal suchas Ta, Nb, or Mo, or stainless steel, is prepared, and a non-oxidizinglayer 21, comprising a noble metal such as Ag, Au, Pt, Ru, Rh, Pd, Os,or Ir or an Ag or Au alloy, is formed on the surface of the core rod 20by a surface treatment method such as a plating method. Thenon-oxidizing layer 21 may be formed by a film-forming method such as aCVD, vapor deposition or sputtering method, or a method of winding anoble metal tape or cladding a noble metal pipe, as described above.

The support 20, which is an elongated reinforcing core material having anon-oxidizing layer 21 formed thereon, is sent to the four-directionroll 10, and the powder material 15 is supplied from the hopper 13 tothe periphery of the non-oxidizing layer 21 and press-bonded thereto bythe four-direction roll 10 to produce a superconducting element wire ora composite wire in which a pressed powder layer 22 is formed on theoutside of the non-oxidizing layer 21.

The thus-obtained element wire is sent to a known induction heater 11 inwhich an oxidizing atmosphere can be maintained so that the wire passingtherethrough can be heated in the oxidizing atmosphere. In the inductionheater 11, the pressed powder layer 22 is subjected to heat treatment inthe oxidizing atmosphere at a temperature of about 700° C. to about1100° C. for about 1 to 100 hours so that a superconducting layer isformed in the pressed powder layer to form a superconducting wire. Inthis heat treatment, since the non-oxidizing layer 21 is interposedbetween the oxidizing metal support 20 and the powder material 15 of thesuperconductor, the oxygen contained in the powder material 15 is notabsorbed by the oxidizing metal support 20 and elements in the powdermaterial 15 are hence reacted at a given component ratio to produce thesuperconducting layer.

After the heat treatment has been conducted for a given time, thesuperconducting wire produced is wound up by the winding machine 12.

In the thus-produced superconducting wire, the oxidizing metal support20 is covered with the non-oxidizing layer 21 so that the oxygencontained in the powder material adequately contributes to theproduction of a superconducting substance, whereby a sufficient amountof a superconducting layer for use in a desired application can beproduced. It is therefore possible to obtain a long superconducting wirehaving a relatively high critical current density and a stable quality.In addition, the pressed powder layer 22 on the outside of the oxidizingmetal support 20 is exposed to the air and thus oxygen is uniformlysupplied over the entire length of the layer during the heat treatmentin the induction heater 11 so that a uniform superconducting layer canbe efficiently produced.

The superconducting wire of this embodiment has the metal support 20 andthe superconducting layer produced on the exterior thereof and thus canbe directly connected to other conductors.

Alternatively, a tubular support can also be used in place of the solidsupport 20. If a tubular support is used and a superconducting layer isproduced on the inner and/or outer periphery of the support to produce asuperconducting wire, a refrigerant is allowed to flow through thesupport so as to cool the superconductor from the inside thereof.

FIG. 3 shows a modification of the superconducting wire shown in FIG. 2.A superconducting wire C has a structure in which the above-describedsuperconducting wire B is covered with an oxidizing metal tube 26, whichalso serves as a stabilizing material, with a non-oxidizing metal layer25 interposed therebetween. The tube 26 is made of the same material asthat of the support 20 of the above-described superconducting wire B,and the non-oxidizing metal layer 25 is made of the same material asthat of the non-oxidizing metal layer 21 of the superconducting wire B.

The superconducting wire C having the above-described structure can beproduced by the method described below. A core material 20 having thenon-oxidizing metal layer 21 formed in its periphery and the tube 26having the non-oxidizing metal layer 25 formed in its internal peripheryare prepared, the core material 20 having the non-oxidizing metal layer21 is concentrically inserted into the tube 26, and a powder material ischarged between the non-oxidizing metal layer 21 and the tube 26. Thesematerials are then subjected to the process of reducing the diameterthen to the heat treatment so as to change the powder material into asuperconductor. Since the non-oxidizing metal layers 21 and 25 areformed in the outer periphery of the core material 20 and the innerperiphery of the tube 26, respectively, there is no danger that theoxygen contained in the powder material will be taken away by the corematerial 20 and the tube 26 during the heat treatment, thus ensuringthat a sufficient amount of superconducting layer is produced.

A superconducting wire can also be produced by providing thenon-oxidizing metal layer 25 in the inner periphery of the tube 26,charging only the superconductor material in the tube, and then heatingthese materials.

FIGS. 4 to 8 show a second embodiment of the present invention. In thisembodiment, a raw material from which the carbon content has beenremoved and which has been temporarily calcined under the sameconditions as those described above is ground well so that the particlesize is made uniform. The particle size is typically smaller than about5 μm, preferably about 0.5 μm to about 3 μm. The raw material having auniform particle size is charged in a mold, such as a rubber mold,having a space of desired shape, and is then pressed by applyinghydraulic pressure such as hydrostatic pressure so as to be suitablymolded to obtain a molded product of any desired shape such as acylindrical or other forms such as a disc-like form. FIG. 4 shows acylindrical molded product 31. The rubber mold used in the molding ispreferably made of a flexible material such as natural rubber or asynthetic rubber. A known hydrostatic press such as a cold hydrostaticpress can be used as the press. The molding pressure of a hydraulicpress depends upon the composition and compositional ratio of the powdermaterial and the size of the molded product, but is typically within therange of from about 1.5 to about 10 ton/cm², preferably from about 1500to about 3500 kg/cm².

The above-described molded product is then subjected to the heattreatment which is performed in a flow of oxygen under such typicalconditions as a temperature of about 800° C. to about 1100° C. and atime of about 1 to 300 hours. This heat treatment causes the oxides ofthe alkali earth metal elements and the elements in Group IIIa of thePeriodic Table to react well with the copper oxide in the molded product31, the whole of which is thus changed into an oxide superconductor of,for example, a layer perovskite form.

The molded product 31 is then coated with a non-oxidizing metal layer31a to form a composite element. The non-oxidizing metal layer 31a canbe made of the same material as the non-oxidizing metal layer 21 inaccordance with any one of the above-described surface treatmentmethods.

A plurality of the thus-formed composite elements are assembled to formthe composite element assembly 32 shown in FIG. 5, which is then coveredwith an oxidizing metal tube 33 made of a material that is harder thanthe material used in the above-described non-oxidizing metal layer 31ato form a composite element assembly 34 with a sheath. The difference inVickers hardness between the metal tube 33 and the metal layer 31a ispreferably from Hv=about 20 to about 200. The composite element assembly34 with a sheath is subjected to a process for reduction of its diameterby a known method to form the wire shown in FIG. 6. When a plurality ofthe molded products 31 (seven products in FIG. 5) each having thenon-oxidizing metal layer formed thereon are assembled as shown in FIG.5, one of the molded products 31 having the non-oxidizing metal layers31a thereon is placed at the center, and the other molded products 31(six) each having the non-oxidizing metal layer 31a are arranged aroundthat molded product 31 at the center. The material used for forming theoxidizing metal tube 33 is properly selected in accordance with thematerial used for forming each of the non-oxidizing metal layers 31a.For example, when each non-oxidizing metal layer 31a is made of Ag,stainless steel such as 18Cr-8Ni (Japanese Industrial Standard SUS304)or 18Cr-12Ni-2.5Mo (Japanese Industrial Standards SUS316) or a Cu-Nialloy is used for the oxidizing metal layer 36.

The metal layer 33 of the wire is then covered with a barrier layer (notshown) such as of Ni and Ti, and then with a stabilizing tube 37 madeof, for example, oxygen free copper, as shown in FIG. 7, and issubjected to the process of wire drawing to produce a desired diameterfollowed by heat treatment to produce the oxide superconducting wire 38shown in FIG. 8. In this case, the stabilizing tube 37 functions tofacilitate the process of wire drawing and to stabilize the finallyobtained oxide superconducting wire. Examples of materials that may beused for forming the stabilizing tube 37 include known metals withexcellent processability such as copper, copper alloys and aluminum. Theheat treatment can be performed under such conditions that thetemperature is substantially the same as the molded product 31, and thetime is substantially the same as that described above. Such wiredrawing and heat treatment enable each of the plurality of the moldedproducts 31 to be uniformly reduced in diameter and heated to form asuperconducting wire of a small diameter.

Since the thus-obtained oxide superconducting wire 38 contains thenon-oxidizing layer 31a which is provided on the outside of each of thesuperconducting fine wires 31b, the non-oxidizing metal layer 31aensures that the oxygen in each of the superconducting fine wires 31b isprevented from being robbed by the cylindrical oxidizing metal layer.The oxide superconducting wire 38 is therefore able to maintain the goodsuperconductivity of the superconducting fine wires 31b containedtherein and thus to exhibit good superconductivity as a whole.

The thickness of the non-oxidizing layer 21, 25 and 31a should be enoughfor preventing oxygen from being taken away by the oxidizing metalsupport from the oxide superconducting material, but is mainly dependson the diameter reducing processing. In the embodiments illustrated, thethickness of the non-oxidizing layer of Ag, before subjected to diameterreducing, is typically larger than about 0.1 mm for the reduction, andthe upper limit is determined in view of cost and may be larger thanabout 5 mm. In the embodiment in FIG. 3, the non-oxidizing layer 21, 25of Ag has a thickness about 2 mm before it is subjected to diameterreducing.

EXAMPLE 1

A reinforcing core material was prepared by forming a 5 μm thickAg-plating layer on the surface of a Cu-Ni alloy wire with an externaldiameter of 5 mm.

A Y₂ O₃ powder, BaCO₃ powder, and CuO powder were mixed so as to have acomposition of YBa₂ Cu₃ O₇, and the mixed powder obtained was heated at700° C. for 3 hours and then calcined by heating at 900° C. for 12hours. The calcined substance obtained was ground to obtain a powdermaterial. The powder material was charged into a hopper having the samestructure as that of the hopper shown in FIG. 1, and the reinforcingcore material was led into the four-direction roll so that the powdermaterial was supplied to the periphery of the reinforcing core materialand pressed thereon to form a pressed powder layer. The thus-obtainedwire was sent to the induction heater in which oxygen was blown at arate of 500 cc/minute so as to continuously produce a flow of oxygenoutside of the pressed powder layer, and the wire was then subjected toa heat treatment at 900° C. for 1.5 hours to form a superconductinglayer in the pressed powder layer, a superconducting wire being therebyobtained.

The obtained superconducting wire showed the transition to asuperconductive state and a critical current density of 500 A/cm² whenbeing cooled with liquid nitrogen.

EXAMPLE 2

A wire was prepared from the same materials and by the same method asthose employed in Example 1, and then subjected to heat treatment at900° C. for 1.5 hours under the same conditions as those employed inExample 1. In the course of cooling of the wire to room temperature byallowing it to stand, it was maintained at about 500° C. for about 24hours to transform the crystal form of the superconducting layer from acubic system to a rhombic system. The alloy wire used was made of aCU-10 wt% Ni alloy, the particle size of the powder of the calcinedsubstance was about 5 μm, and the thickness of the pressed powder layerwas about 2 mm.

The superconducting wire obtained showed a critical temperature of 91 Kand a critical current density of 500 A/cm² at the temperature of liquidnitrogen in a zero magnetic field.

EXAMPLE 3

Powders of Y₂ O₃, BaCO₃ and CuO were mixed in a composition ratio of Y:Ba: Cu: O=1:2:3:7 to form a mixed powder. The mixed powder obtained wasthen subjected to preliminary calcination at 700° C. for 3 hours andthen to temporary calcination at 900° C. for 12 hours. The mixed powderwas then ground into a fine particle powder and pressed by a rubberpress to obtain a cylindrical molded product having an external diameterof about 10 mm.

The molded product obtained was then inserted into a silver tube havingan external diameter of about 15 mm and a wall thickness of 2 mm, andthe entire diameter of the product was then reduced to form a wirehaving an external diameter of about 4 mm.

Seven such wires were prepared, and one of the seven wires was placed atthe center while the other six wires were arranged around the wire atthe center to form an assembly. The resulting assembly was then insertedinto a tube made of 18Cr-8Ni stainless steel (Japanese IndustrialStandard SUS304) and having an external diameter of about 14 mm and awall thickness of 0.5 mm, and then inserted into a copper tube having anexternal diameter of about 20 mm and a wall thickness of 2.5 mm to forma composite assembly. This composite assembly obtained was subjected toa process of wire drawing to reduce its diameter to about 1.5 mm, andthen subjected to heat treatment to obtain an oxide superconducting wireof a multi-strand form. This heat treatment was performed at atemperature of 900° C. for 3 hours.

The measured critical temperature of the thus-obtained oxidesuperconducting wire was about 90° K., and the critical current densitywas about 500 A/Cm² at the temperature of liquid nitrogen. It was thusfound that the oxide superconducting exhibits good superconductivity.

EXAMPLE 4

A multi-strand wire was prepared using the same materials and under thesame conditions as those employed in Example 3, and it was alsosubjected to a process of wire drawing and was then subjected to a heattreatment at 900° C. for 3 hours. In the course of cooling of theprepared wire by allowing it to stand, it was maintained at 500° C. for24 hours to transform the form of the produced superconducting crystalsfrom a cubic system to a rhombic system. In this example, the particlesize of the calcined ground powder was about 5 μm, the hydrostaticpressure of the rubber press in which the calcined ground powder wascharged was 2500 kg/cm², the hardness of the silver tube and thestainless steel tube were Hv=30, 150, respectively, and the thickness ofthe silver layer provided between the respective strands ofsuperconducting wire was 0.2 mm.

The obtained superconducting wire showed a critical temperature of 89°K. and a critical current density of 500 A/cm² at 77° K. in a zeromagnetic field.

What is claimed is:
 1. An oxide superconducting wire comprising:anoxidizing core extending along the wire, a first non-oxidizingintermediate layer formed on a surface of the oxidizing core to preventoxygen atoms from passing therethrough, an oxide superconducting layerformed on a surface of the first non-oxidizing intermediate layer, asecond non-oxidizing intermediate layer formed on a surface of the oxidesuperconducting layer to prevent oxygen atoms from passing therethrough,and an oxidizing metal support layer formed on a surface of the secondnon-oxidizing intermediate layer, the oxidizing core being in the formof an elongated rod of an oxidizing metal material selected from thegroup consisting of Cu, copper alloy, Ta, Nb, Mo, and stainless steel,the oxidizing metal support layer being in the form of an elongatedmetal tube of an oxidizing metal material selected from the groupconsisting of Cu, copper alloy, Ta, Nb, Mo, and stainless steel.
 2. Anoxide superconducting wire according to claim 1, wherein the firstintermediate layer and the second intermediate layer are composed of anoble metal selected from the group including Ag, Au, Pt, Ru, Rh, Pd,Os, Ir, silver alloys and gold alloys.